Metal halide gas discharge lamps include an arc tube containing vaporizable material including metal halide and mercury, and a pair of spaced electrodes made, e.g., of tungsten. To start such a lamp, a voltage of typically several hundred volts is impressed across the electrodes with the object of creating an arc discharge from one electrode to the other. During the initial, low pressure stage of starting a metal halide lamp from normal ambient temperature (e.g., 25.degree. C.), a high electric field gradient exists in the proximity of the electrodes. The high gradient results in ions of relatively low atomic weight, such as argon, striking the electrodes at high speed, and dislodging tungsten from the electrode, a process known as sputtering.
The dislodged particles of tungsten accumulate on, and darken, the inner wall of the arc tube, degrading the lumen maintenance of the lamp. The present inventors have considered a possible approach to reducing the amount of electrode sputtering by elevating the temperature of the arc tube before attempting to strike an arc in the tube. Increasing temperature increases the vapor pressure of vaporizable material in the arc tube. This reduces the mean free path of the ions, reducing their impact velocity at the electrodes and thereby reducing the sputtering damage. Additionally, when material with a high atomic weight, such as mercury, becomes vaporized, the ions of such material are accelerated less by the electric field gradient in relation to low atomic weight material, and thus impact the electrodes at a relatively lower velocity. This additionally reduces electrode sputtering.
The present inventors considered adding a heater to heat the arc tube and thereby increase its pressure prior to attempting to strike an arc in the tube. A problem faced by the present inventors was how to avoid short lamp life due to the so-called photoelectron effect. According to this effect, energetic photons from the arc tube impinge upon a typical, metallic heating filament, for instance, and cause the release of electrons therefrom. Such "photoelectrons" accumulate on the outer wall of the arc tube, creating a negative electric field, and induce sodium ions to migrate out of the arc tube. The loss of sodium rapidly shortens lamp life.